Calcium is known to be subject to matrix interference within the plasma, due to nearby vaporizing aerosol droplets.
"Incompletely Desolvated Droplets in ArgonIinductively Coupled Plasmas: Their number, original size and effect on emission intensities.", John W. Olesik, Julius C. Fister III, Spectrochemica Acta Part B: Atomic Spectroscopy, Volume 46, Issues 6-7, pages 851-868.
I'm assuming that your problem of reproducibility is related to matrix spike recoveries, not calibration drift observed when analyzing a check standard. According to Olesik and Fister, Ca I (atom) emission intensities are enhanced near vaporizing droplets, whereas Ca II (ion) emissions are depressed. The size of incompletely desolvated aerosol droplets in the plasma is a function of the applied power and nebulizer gas flow rate. Higher power and lower neb flow rate produce a more robust plasma, which gives more complete desolvation of the aerosol droplets. Use of an internal standard can also help to correct for matrix and/or transport effects.
City of Fresno Wastewater Laboratory
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Cory Larsen
Sent: Tuesday, March 21, 2017 9:02 AM
To: [log in to unmask]
Subject: Variability of Ca with ICP-AES
We are having problems with reproducibility of calcium on our P-E Optima 2100 ICP-OES. Most elements do well to within +/- 3-4 %, but we can't get calcium to closer than +/- 10%.
Do we need an internal standard? Do we need an ionization buffer? My understanding is that buffers are mostly for alkali metals in axial viewing mode. Calcium is an alkaline earth, of course, and we look at it in radial mode.
Any suggestions? Thanks.
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